1. Field of the Invention
The present invention relates to an application method of a liquid material using spin coating, application equipment for such application method, a product which is produced using such application method (for example, an optical disc, a liquid crystal display, a magnetic disk, etc.) and a production process and a production apparatus of such product using such application method.
2. Description of Related Art
The conventional methods of applying a liquid material are shown in FIGS. 8(a) to (d) wherein the liquid material 804 is applied on a substrate 806 using a brush 802 (see FIG. 8 (a)), a liquid material 804 is dropped onto a substrate 806 from a nozzle 808 (see FIG. 8 (b)), a liquid material 804 is sprayed onto a substrate 806 through a nozzle 810 (see FIG. 8 (c)), and a liquid material 804 on a roller 812 is transferred to a substrate 806 (see FIG. 8 (d)).
In order to remove an amount of a liquid material that has been excessively applied on an object, to make a thickness of a coating formed on an object by the application, or to uniformly spread on an object a liquid material which is supplied thereto, a spin coating method is used wherein the object to which the liquid material is to be applied is placed on a turn table (or a stage), which is, in turn, rotated so that a centrifugal force is applied to the liquid material. Generally, the uniform thickness of the coating is obtained by the spin coating method only when the thickness of the coating is small enough. When the thickness of the coating is large, the variation over the applied surface is large after the spin coating. In a case where the thickness of the coating is 10 micrometers or more, the thickness variation of the coating is 20% or more based on an average thickness.
FIGS. 9(a) and (b) show in a sectional view a typical state of the liquid material 903 supplied to a plate 901 supported on a stage 902 just after the liquid material has been applied using the conventional spin coating method. With the conventional spin coating method, a bulged portion 904 of a coating thickness is formed on an outer peripheral part 910 of the plate 901 (i.e., a portion of the plate shown with a bi-directional arrow, see FIG. 9(a)). Moreover, in the case where the liquid material is applied to a plate 901 that has a hole 908 in its central part, a thickness of the coating on a portion 920 of the plate around the hole 908 (i.e., a portion of the plate shown with a bidirectional arrow, see FIG. 9 (b)) becomes thin.
As a product produced using such a spin coating method, there is an optical disc which has been recently used as a mass recording medium. Attempts to increase the capacity and high recording density of the optical disc are on-going now, and specifically, an approach of enlarging a numerical aperture (NA) of an object lens for a record/reproduction device, and an approach of shortening a wavelength of a laser beam to be used are now being studied. In the optical disc, when a thickness of a substrate on a record/reproduction side of the disc, the substrate functioning as a light permeable layer (i.e. a substrate on a side from which a laser beam is injected), is thinner, an effect of aberration on a laser beam spot becomes smaller so that a tolerance of an inclination angle (tilt) of the disc can be increased. Based on this, it is proposed that the thickness of the substrate on the record/reproduction side is about 0.1 mm, NA is about 0.85 and the wavelength of the laser is about 400 nm. With such a proposal, it is desirable that the variation in the thickness of the substrate on the record/reproduction side be suppressed to within 5% considering the effects of a record/reproduction beam on the focus and spherical aberration.
As a method which achieves such suppressed variation in the thickness, it is conceived that sheet form substrates, each having a uniform thickness (of about several tens of micrometers), are laminated with a light curable resin. However, the use of such sheet form substrates is very costly, and thus, it is preferable to form the record/reproduction side substrate by only using the spin coating method.
It is very difficult to apply a liquid material by the conventional spin coating method so as to form a uniform coating having a thickness is not smaller than several tens of micrometers. According to the conventional spin coating method, the liquid material is always forced toward a periphery of the object due to the centrifugal force, so that the thickness of the coating formed by the application is thinner toward an inner peripheral part of the object and thicker toward an outer peripheral part of the object. Moreover, the thickness of the coating of the outer peripheral part becomes extremely thick because of the surface tension of the liquid material.